Slide-wire unit for potentiometers



Sept. 29', 1936. o. H. HUNT 2,055,766

SLIDE WIRE UNIT FOR POTENTIOMETERS Filed June 29, 1934 INVENTOR 02/?0 H.Hu/vT ATTORNEY Patented Sept. 29, 1936 SLIDE-WIRE UNIT FORPOTENTIOMETERS Ozrc H. Hunt, Waterbury, Conn., assignor to The BristolCompany, Waterbury, Conn., a corporation of Connecticut Application June29, 1934, Serial No. 733,086

1 Claim. (Cl. 201-62) The invention relates to electrical measuringand/or controlling apparatus of the potentiometer or null type and moreparticularly to the slide-wire unit customarily utilized therein andcomprising a length of resistance wire or slidewire element and a slideror contact-making member adapted for sliding engagement thereover.

The ideal slide-wire for a self-balancing po tentiometer, for example,consists of a bare wire of resistance metal so disposed that the slidingcontact may engage any point on its length. Owing, however, to thephysical diftlculties encountered in obtaining a suitable resistancevalue within the available dimensional limits, it is seldom that thisideal can be attained with out recourse to relatively complicatedmechanisms, which themselves tend to introduce errors ofisetting much ofthe precision attendant on the use of the continuous bare wire.

In potentiometers of the self-balancing type it is usual, therefore, forthe slide-wire to take the form or" a continuous winding on a mandrel orcard, a strip of the wire along one edge being left bare for contactwith the slider at successive turns, as hereinbefore set forth. By theuse of a suihciently great number of turns of wire, the potential dropbetween successive convolutions may be made so small that a single stepfrom turn to turn, or a momentary shortcircuit between turns isimperceptible in its effect on the galvanometer of the potentiometer.

It will be appreciated, however, that in a slide-wire of this nature,there are certain limitations, and that a compromise must frequently bemade between the most desirable resistance value and the size of wiresuitable for the purpose of contacting. In order to have a smoothpotential gradient there must be employed a relatively great number ofturns, calling for a fine wire and a consequently high resistance value.While the total ohmic value of the slidewire circuit may be reduced byplacing a suitable resistance spool in parallel with the slidewire, theresistance of the galvanometer circuit is not thereby reduced inproportion, and sensitivity will consequently be sacrificed. Again,slight irregularities in the winding, or particles of foreign matterupon the surface of the wire, may affect the contact resistance or mayintroduce actual open circuits into the galvanometer circuit.

It is an object of the present invention to overcome the aforesaidlimitations and to provide a slide-wire unit which shall be positive inoperation and of improved reliability against errors or failure due tothe contact resistance or to open circuits; also, a unit which shall beof rugged and rigid construction.

The invention has for a further object the 5 provision of a resistanceelement which, while retaining the advantages of a large number ofcontact points, shall be adapted to a low-resistance potentiometercircuit.

In carrying out the invention, the slide-wire 10 structure is subdividedinto a plurality of circuits electrically in parallel, and having theirrespective sliding contacts at the same potential, a single slider orcontact-making member, for example, bridging the parallel-connected 15slide-wires.

By this expedient, not only is the total resistance reduced to a valuelower than practicable with a single slide-wire, but the probability oftrouble due to contact resistance, open circuits, or irregularity ofwinding is reduced in direct proportion to the number of parallelcircuits.

The nature of the invention, however, will best be understood whendescribed in connection with the accompanying drawing, in which:

Fig. 1 illustrates, in perspective and more or less diagrammatically, apotentiometer assembly illustrating the construction and arrangement ofthe novel slide-wire unit therein.

Fig. 2 is a detail view, on an enlarged scale, of a suitable form of theslide-wire unit; and Fig. 3 is an end view thereof.

Referring to the drawing, more particularly Fig. 1 thereof, twocalibrated slide-wire elements, as the cards It and II bearing therespective windings l2 and I3, are indicated as disposed adjacently andsubstantially parallel to each other and to be out of electrical contactexcept at their respective ends where they are joined together by thewires l4, l5 and l6, [1. These ends are then electrically connected intothe usual potentiometer circuit including a battery 26 which suppliescurrent to the regulating rheostat 2! through the two parallel connectedslide-wire elements, whereby the potential drop along both of saidelements is the same and any two opposite points on the respective cardswill be at the same potential.

A source of controlling electromotive force or force to be measured,such as that derived from the thermocouple 25, is arranged to be appliedto one end of the combined slide-wires and to one side of a galvanometer26, the other side of the galvanometer being connected to two sliderelements or contact members 21 and 28 adapted 55 for engagement with thecorresponding windings l2 and I3 for completing the potentiometercircuit.

' It will be understood that these slider contacts, corresponding innumber to the individual slide-wire elements, will be electricallyconnected together in such a manner that there can exist no potentialbetween the various contacts. The latter are supported upon a movablecarriage 30, flexible leads 3! and 32 being included in the connectionto the contacts 21 and 28 to admit of reciprocation of the saidcarriage. This may be effected from a lead screw 33 engaged by a nut ofthe carriage, the lead screw being under control of a servo-motor fi lgeared to the said lead screw and in turn under control of thegalvanometer 26. For example, the control may be efiected through thecontact system 35 actuated under conditions of unbalance of potential inthe galvanometer circuit which cause deflection of the galvanometer inthe sense to afiect the motor so that the carriage 30, together withcontacts '21 and 28 borne thereby, is moved by the lead screw 33 in adirection as to introduce into the galvanometer circuit from theslide-wires a pote'ntialtendin-g tobalance this circuit and to returnthe galvanometer to its neutral position all of which is well understoodand forms no particular part of the present invention. The carendportion of each card and the said bar 32.

One or these bolts is placed at each end of the card; and there isspaced along the bar 42, more or less uniformly, a number of hollowposts 46 of insulating material and of a diameter substantially equal tothe space between the windings. r

7 Supported by these posts are 'U -shaped clamping members 4'! embracingthe upper side of the windings, being held in place by screws 48 passingthrough the hollow posts 46 and tapped into the bar 42. This arrangementaiiords a structure which is rigid against lateral distortion as well asagainst bending stresses due to the contacts bearing upon the loweredges of the said windmgs.

Directly beneath these windings and carried upon rails as and 50 ismounted to run a carriage 5! which bears two mechanically independentsliding contacts 52 and 53 for engagement with the lower edges of thewinding which are left bare for securing electrical contact thereto.Leaf springs 54 and 55, resting upon the body of the carriage are causedto bear against the contacts 52 and 53 to hold the same in properengagement with the respective windings.

It is understood, of course, that the two windings are exactly similar;and it will be appreciated, furthermore, that the total slide-wireresistance in this novel arrangement is less than that of a singleslide-wire so that the resistance of the galvanometer circuit may bereduced to the same degree as that of the slide-wire circuit. Moreover,errors in measurement, due to irregurarity in individual units, areminimized; and a high resistance, or open circuit, .at one contact willnot prevent a definite and accurate measurement from being obtained.

I'ola im:

A mounting for slide-wire resistors of the type wherein a continuouswire of resistance, material is wound on a fiat card and adapted to beerrgaged on an edge by a sliding 'con'tactor: said mounting comprisingan elongated metal bar to the opposite sides of which are affixed two ofsaid cards in parallel relationship, together with clamping meansintermediate the extremities of said cards and partially including thesame, insulating spacer means located along the one edge of the bar andbetween the corresponding projecting portions of the respective cards,and bolts passing through the separating means into the bar to securesaid clamping means thereto whereby the clamping means will resistedgewise strain on said-cards, and obviate distortion of the same.

OZRO H. HUNT.

